International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol. 2 Issue 4 Dec - 2012 1-12 © TJPRC Pvt. Ltd.,
SPLIT BEAM LOW FORCE LOW LOSS CAPACITIVE HIGH FREQUENCY MEMS SWITCH ON QUARTZ FOR RECONFIGURABLE CIRCUITS 1, 2
ANESH K SHARMA, 1ASHU K GAUTAM, 1CG BALAJI, 2ASUDEB DUTTA & 2SG SINGH 1
Directorate of Radar Seekers, Research Centre Imarat, Hyderabad, India 2
Indian Institute of Technology Hyderabad, India
ABSTRACT This paper presents the novel capacitive high frequency MEMS switch design and fabrication in broad frequency band for reconfigurable circuits. This switch was designed for symmetric actuation so as the deflection is uniform with a very low pull-in force but having sufficient restoring force for stable operation. A unique single bias pad is used to bias the two actuation pads in order to provide the simplicity during implementation in system. In this work, capacitive shunt switch was developed using the customized process flow. The membrane structure was split into small strips to achieve low stress in the beam, complete sacrificial removal for good RF characteristics and minimization of the squeeze film damping effects. This configuration was developed for high spring constant keeping in view the restoring force requirement for smooth operation of the switch. The Quartz substrate was used for fabrication to exploit its properties for high frequency operation. As a result a low actuation voltage of 24.6V was achieved with an insertion loss of 0.24dB and return loss better than 22dB in the frequency range of 4-20GHz. The isolation of 37dB was achieved in the off state.
KEYWORDS: Reconfigurable, Isolation, Restoration Force, Spring Constant, Electrode, Actuation Voltage, Phased Array
INTRODUCTION Reconfigurable circuits [1] are gaining significance in the field of communication, radar and the navigation applications. Lot of emphasis is paid for study and development of these using Microelectromechanical systems (MEMS) switches for reconfigurable circuits like tunable filters, switching matrix and Phased array reconfigurable antenna applications [2]. The high frequency MEMS (RF MEMS) switch is defined as a contact switch fabricated using micromachining technology. It possesses the excellent features like low insertion loss due to low ON resistance and large isolation due to small parasitic in comparison to the PIN diode and MESFET switches [3]-[6]. Additionally these provide benefits such as reduced power dissipation, noise, weight, size and cost etc. Major problem associated with RF MEMS stable operation is the degradation of contact between the actuation electrode and the bridge. This is mostly due to the contamination and the electromigration of materials across the contact, the creeps, the ductile, and the brittle wearing of the contact and the hardening of the contact area. However this issue was addressed by some researchers by designing the device such that the actuation voltages do not exceed 50 V [7]. In capacitive switches, the metal-to metal contact is eliminated by covering the actuation pad and rf line with a dielectric layer to reduce the stiction effects, which further offers the improvement in ratio of switch capacitance in down and up states (Cdown/Cup) [8]. However, dielectric layer usually undergoes charging effects which can cause stiction phenomena between the dielectric layer and the bridge metal causing variation in the pull-down voltage [9]. In spite of some active research in this area, stable switching of signals is a problem which stems from the insufficient restoration force [10]. These necessitate examining some new design and process of these devices. In this paper a novel configuration design leading to good “off-to-on” capacitance ratio with low
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Anesh K Sharma, Ashu K Gautam, CG Balaji, Asudeb Dutta & SG Singh
insertion loss and low actuation voltage with split beams without affecting switch’s other performance parameters has been proposed. A summary of the capacitive switches reported in literature is listed in Table I [11]-[16]. Table 1: Summary of Reported Capacitive Shunt Switches Comparison
Dielectric (Å) Si3Ni4 (1500) Si3Ni4 (1000) Si3Ni4 (2500) Si3Ni4 (2500) Si3Ni4 (2000) Si3Ni4 (1500)
Vact(volts)
tswitch(µs)
Coff(pF)
Ref
1-3
Cdown/Cup Ratio 30-50
6-20
20-40
12-25
6-15
0.5-3
20-40
13
20
250
2-6
N/A
14
110
N/A
2.3
28.75
15
21-24
N/A
4.8
36.09
16
30
N/A
0.27
27
17
12
The hysteresis response was studied in detail so as to provide the restoration force capable of stable operation. This analysis is important as the RF performance is determined by the mechanical behaviour of the MEMS switch. High frequency simulations were carried out using High frequency structure simulator (HFSS) for two different mechanical states. The contact region was modelled as the RLC circuit. In this work, the design of the switch, manufacturing process and analytical results have been discussed which approves the superiority of these RF MEMS switches for specific applications. The paper is organized as follows. Section II introduces the simple but efficient design, simulation and behavioural model of the RF MEMS switches taken up for fabrication. Section III presents the fabrication details and flow followed during fabrication. Section IV presents the details of the inspections carried out during and after fabrication. Section V summarizes the experimental results. Section VI finally concludes with overall results.
DESIGN AND SIMULATION The primary emphasis of this work was to design the capacitive shunt configuration of MEMS switch [17]-[19] with membranes for low force with electrostatic actuation. This configuration has bias pad on the periphery in view of smaller length of bond wire during assembly and packaging to avoid the parasitic effects at high frequencies. The design of this switch was done using split beams in rectangular shape instead of holes from other reported work which makes this design unique. This configuration in comparison to the standard rectangular holes has the advantage of low stress across the beam making it more robust to the environmental effects and ease in structure release due to continuous larger area for etchant percolation. The Fig. 1(a) and Fig. 1 (b) shows the 3D models of the split beam and rectangular holes analysed with FEM for stress analysis. The Beam outer dimensions have been taken as same for both the configurations except for the internal structural design. The mass of the beam structure has been maintained same for both the configurations so as to analyse the proposed split beam structure with respect to the rectangular holes structure. The holes dimension is 10x10 µm2 and the edge to edge gap between the holes has been optimized in order to achieve the same mass as of the split beam structure. The split beam details are given subsequently. The analysis shows that the stress of the order 630 MPa in case of rectangular holes and 380 MPa for the split beam configuration has been observed. The stress analysis shows the superiority of split beam design with respect to the rectangular holes.
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Split Beam Low Force Low Loss Capacitive High Frequency Mems Switch on Quartz for Reconfigurable Circuits
(a)
(b)
Fig. 1: (a) snd Fig. 1(b) Shows the Stress Analysis for the Split and Rectangular Holes Beam Respectively Fig. 2(a) shows the top view of the total configuration and Fig. 2(b) shows the cross sectional view. The Coplanar Waveguide (CPW) of 50Ω impedance with Ground/Signal/Ground (GSG) configuration was used for design. The G/S/G dimensions for the CPW configuration are 19/200/19 µm. The air gap of 2.9µm was maintained for this design. This has the dielectric on actuation pads as well as on the rf line.
(a)
(b)
Fig. 2: Top View of (a) Total Configuration and (b) Cross Section for Beams Area Having Actuation Pad The CPW metal thickness was taken as 3.0µm. Fig. 3(a) and Fig. 3(b) shows the detailed dimensions of the first metal layer layout and zoomed view of the actuation electrodes respectively.
(a)
(b)
Fig. 3: (a) Shows the Details of the First Metal Layer, Fig. 3(b) Shows the Details of the Actuation Electrodes For the proposed structure, the 3D full wave electromagnetic simulation was carried out by a finite element method using Ansoft HFSS. Fig. 4 shows the effective length of the shunt membrane and the points at which pull in force
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Anesh K Sharma, Ashu K Gautam, CG Balaji, Asudeb Dutta & SG Singh
is applied due to underneath placement of actuation electrodes. The effective beam has length of 396µm. The EM simulation results of RF parameters are shown in comparison to the measured results in the experimental results and discussion section V.
Fig. 4: Dimensions for the Reduced Model Excluding Anchors A 3D model in Coventorware is generated using customized process. Device fabrication method is implemented for the structure feasibility. A 2D layout is generated for each and every mask pertaining to the fabrication steps of the device. Finally both, the fabrication flow and 2D mask layouts are integrated to generate the 3D model of the device. The analysis has been done using finite element method using manhattan meshing and the beam is fixed at the extreme ends. Further the membrane is allowed to deform with respect to the application of gradual increase in voltage. The electro mechanical simulation result for the pull in voltage is shown in the Fig. 5. A nearly linear deflection has been obtained for a travel of 1.6µm at a voltage of 22.2V up to the pull in point. Beyond the pull in voltage point a sudden snap down was observed and analyzed under contact and hysteresis analysis.
Fig. 5: Pull in, Contact and Hysteresis Analysis Curve. The Simulations were Carried out Using FEM Model of the Switch. The Hysteresis Curve Shows the Pull Back of the Beam after Removal of the Actuation Voltage The membrane was split into ten strips each having width of 15µm and gap of 11µm. This was optimized keeping in view the air damping and the easy removal of the sacrificial layer during the fabrication of MEMS switches. The pull in voltage was calculated using following equation (1). (1) Where k = Spring Constant d = gap between electrodes = 2.9 µm A = Area of electrode ε = permittivity of air Spring constant has been calculated using equation (2) for the membrane.
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Split Beam Low Force Low Loss Capacitive High Frequency Mems Switch on Quartz for Reconfigurable Circuits
(2) Where t = Thickness of membrane = 1.5 µm E= Young’s Modulus = 79 GPa W = width of the beam Based on the model values used in equation (2), the spring constant was found to 17.17 N/m for the membrane. This value was inserted in equation (1) and the pull in voltage was calculated as 20.76V. By simulation the pull in voltage is found to be 22.2V [20]-[21]. The membranes were analyzed for the hysteresis response which is quite crucial for the smooth operation of the switch. This analysis estimates the spring constant responsible for the membrane restoration to the original position. Fig. 5 also shows the contact and hysteresis curve after removal of the DC bias from the actuation pad and the reversal of membrane to its original position under the restoration force at 17V with a contact voltage of 24.1V having air gap as 2.9µ m. A 3D view of the Membrane deflection is shown in the Fig. 6. The maximum deflection has been observed at the centre of the membrane which is required for the proper contact at the centre of the transmission line.
Fig. 6: 3D Model of Beam Under Pull in and Contact Analysis The mechanical resonance is significant from the structural integrity point of view. Membrane was split into smaller widths with uniform air gap between the split membranes. This was introduced to increase the mechanical resonant frequency, ease the sacrificial removal during fabrication for structural release and to minimize the effects of squeeze film damping. The membrane was subjected to modal analysis to estimate the resonant frequency so as to analyze the effect of vibrations when implemented in field applications. The results are shown in the Table II for the six modes. The structural resonance of the first mode was found to be 16.84 KHz through the modal analysis. This value is quite satisfactory in response to the environmental effects this could face in the field applications. Table 2: Modal Analysis for Resonance Frequency Mode 1
Frequency (KHz) 16.84
Mode Mass (kg.) 6.95x10-10
2 3
22.46 37.38
3.12x10-10 3.31x10-10
4
47.69
7.47x10-10
5
50.13
3.80x10-10
6
55.22
4.15x10-10
The dynamic response was carried out using Intellisuite finite element analysis. The switching time can be computed by the equation (3) [22]. It has been observed that by keeping more gaps between the strips, membrane response time can be improved [23]-[24].
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Anesh K Sharma, Ashu K Gautam, CG Balaji, Asudeb Dutta & SG Singh
(3) Whereas Vp is the pull in voltage, Vs is the source voltage and fo is the resonant frequency.. From the simulation ON-time (downward transition) is 32 µsec and OFF-time (upward) transition was found to be 48µsec as shown in the Fig.7 [25]. A potential of 24V was applied to switch on the device.
Fig. 7: Transition Analysis for On / Off Time
FABRICATION The fabrication of the switches was carried out using surface micromachining techniques. Process was customized according to the design requirements. A four mask batch process was used to fabricate the micromechanical switches. The fabrication flow is shown in Fig. 8 (a-f). The quartz substrate of 525µ m thickness with 4 inch diameter was taken for fabrication. The first metal (Au) of 3.0µm thickness for CPW was deposited through E-beam evaporation and patterned as shown in Fig. 8(b) using masks 1. In order to avoid metal-metal contact a Si3N4 (Silicon nitride) layer of 0.1µm was deposited using PECVD and patterned over the transmission line and the actuation pads as shown in Fig. 8(c) using Mask 2. The 3.5µm of photo resist above the CPW metal was coated using spin coating in two steps. A novel technique of photo resist planarization was implemented to avoid the beam deposition on uneven surface [26]-[27]. Mask 3 was used for opening through the sacrificial layer for the beam anchor. The opening is shown in the sacrificial layer in Fig. 8(d). The beam structure with 1.5µm thickness was realized through RF sputter deposition in Fig. 8(e). At the last sacrificial layer was ashed out using plasma etching and release of the switch is shown in the Fig. 8(f).
Fig. 8: (a) to (f) Shows the Fabrication flow Followed During the Switch Fabrication
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Split Beam Low Force Low Loss Capacitive High Frequency Mems Switch on Quartz for Reconfigurable Circuits
INSPECTION The air gap measurement was carried out by non contact methodology using LEXT OLS 3100 confocal laser scanning microscope. The measured device is shown in the Fig. 9. This was scanned across the width of the membrane and it shows the individual step height of the split beam. The results were observed in consonance to the design and fabrication.
9(a)
9(b)
Fig. 9: Air Gap Measurement of Beam from the Bottom Electrode In Non Contact Mode Using The LEXT OLS 3100 Confocal Laser Scanning Microscope. X And Y Axis Shows the Width and Length Respectively. 9(A) Shows the Scan Direction for the Beam. The Split Beams were Scanned across the Width So as to Confirm the Planarity. 9(B) Shows the Gap Height of the Beam from the Bottom Metal Including the Beam Thickness. The Air Gap was Observed As 2.84 µm from the Bottom Electrode as against the Z Axis.
SEM inspection was carried out to inspect and analyze the surface topology of the fabricated device. Fig. 10(a) shows the surface view of the complete device while Fig. 10(b) shows the zoomed SEM view of the membrane area.
Fig.10: (a) SEM View of the Full Device and Fig.10 (b) Zoomed SEM View of the Membrane Area This analysis has given clear view of the edge definition and interspacing of the split membranes. SEM inspection also revealed the complete removal of the sacrificial layer enabling the device to properly perform electrically during the deflected state.
EXPERIMENTAL RESULTS AND DISCUSSIONS DC Characteristics Cup and Cdown i.e. Capacitance in upstate and deflected state was measured at 1MHz using the semiconductor parametric analyzer B1500 of the Agilent. The capacitance ratio (Cdown/Cup) had been measured in the range of 30-35. The absolute measured values are given in Table III. RF Characteristics Two port on wafer RF measurements for the ‘S’ parameters were carried out from 4-20 GHz. The semiautomatic RF probe station of Cascade Microtech Summit 11000 series with Vector Network Analyzer E8363B of Agilent make was used to characterize the switch device. The short open load thru (SOLT) technique was used to calibrate the test set up. A
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Anesh K Sharma, Ashu K Gautam, CG Balaji, Asudeb Dutta & SG Singh
separate standard impedance substrate was used for calibration. The measurement accuracy is traceable to international standards. Fig. 11(a) shows the switch measurement view which has dc bias electrodes on both sides having symmetrical pull in arrangement.
Fig. 11: (a) Fabricated Switch Measurement Set up Showing the CPW GSG Configuration Measured with 200 Micron Pitch Probe The S parameter measurement of the switch has insertion loss of 0.24 dB and return loss better than 22dB in upstate from 4-20 GHz as shown in Fig. 11(b). In the down state switch has isolation better than 37 dB as shown in the Fig. 11(c).
Fig. 11: (b) Shows S Parameter Results in Upstate of the Switch. Measured Results of the Return Loss and Isolation are Compared against the Simulated Results (OFF State)
Fig. 11: (c) Shows S Parameter Results in Downstate of the Switch. Measured Results of the Return and Insertion Loss are Compared against the Simulated Results (ON State)
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Split Beam Low Force Low Loss Capacitive High Frequency Mems Switch on Quartz for Reconfigurable Circuits
The switch shows a good short circuit to ground in the down state [28-29]. The measured parameters show a very good consonance with the simulated results. The parameters of the designed, simulated, fabricated and measured switch are listed in the Table III. Table 3: Summary of Parameters for the Developed Shunt Switch Parameter Length[µm] Width[µm] Height [µm] Membrane Layer Thickness [µm]
Value 396 249 2.84(measured) Au 1.5
Spring Constant[N/M] (Calculated value) Effective mass [Kg] Density of Material [Kg/m3] Mechanical Resonance frequency [KHz]
17.17 18.557 x10-10 19,320 15.31(calculated) 16.84(simulated)
Parameter Sacrificial layer Cantilever Type Dielectric(Å) Actuation area[µm2] Actuation voltage[V] (measured) Switch time[µs] (simulated) Cu[fF] (measured) Cd[pF] (measured) Loss[dB] (measured) Isolation[dB] (measured)
Value Photoresist Split beams 1000 250x130 (x2) 24.6 30-50 231.5 6.96 0.24 >37
DISCUSSIONS RF MEMS switch has been fabricated in capacitive shunt configuration successfully. The switch has performed in the wide frequency band of 4-20GHz as dielectric was used on the rf line. The lower actuation voltage of the order of 24.6V, very close to the simulated values, has been achieved with good restoration action. The developed switch though having two actuation pads is operated with a single bias pad making it appropriate for system implementation. These results make these switches suitable for the reconfigurable circuit applications.
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